Improved INI file search logic
The application now tries several locations, in order:
- Environment variable TFTP_INI (explicit file path).
- Environment variable TFTPD32_INI_DIR (directory path).
- %APPDATA% directory (per-user roaming settings).
- %LOCALAPPDATA% directory (per-user local settings).
- %PROGRAMDATA% directory (system-wide settings).
- Application directory (fallback, same as previous versions).
This allows non-admin users to change settings in their profile,
while administrators can still enforce a global configuration in ProgramData.